E.L.4 lymphosarcoma (E.L.4) s.c. as a syngeneic tumor was transplanted into C57BL/6 mice, 4 weeks old. The tumor growth was more rapid in male mice. An i.p. injection of alpha-MPG or DPA caused a decrease of the tumor growth and prolongation of the survival time in male mice. In contrast, an acceleration of the growth and a reduction of the survival time were observed in females dosed with alpha-MPG or DPA. Such a sexual difference and the activity of drugs decreased with the progress of age. alpha-MPG and DPA had little effect on the number of recovered and living E.L.4 cells in vitro. However, with the inoculation of alpha-MPG-treated E.L.4 into mice, the tumor rapidly grew in a dose dependent manner. On the other hand, there was a tendency toward suppression of the tumor growth when DPA was given. Development and growth of tumor in mice induced by 20-methylcholanthrene showed a tendency toward suppression in male mice when alpha-MPG in a dose of 5 mg/kg was given every other day, and in both sexes with the same dose of DPA. Responses of spleen and lymph node cells to phytohemagglutinin-P (PHA) or lipopolysaccharide (LPS) were reduced by the transplantation of E.L.4 in 4 week old mice. With alpha-MPG and DPA a reduction in PHA response of both the cells tended to recover, and that in LPS response of lymph node cells was recovered. In these mice, the decrease in cytotoxicity of spleen and lymph node cells against E.L.4 was recovered by treatment with both drugs. There was also a complement-dependent cytotoxicity against E.L.4 in their sera, and the activity was further increased by the administration of alpha-MPG. These findings suggest that alpha-MPG and DPA are anti-tumor agents which act through immune mechanisms.